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4.2.3 Server System Variables - MySQL

If you need to administer MySQL, this article gets you off to a good start. The first of a multi-part series, it is excerpted from chapter four of the book MySQL Administrator's Guide, written by Paul Dubois (Sams; ISBN: 0672326345).

TABLE OF CONTENTS:
  1. MySQL Database Administration
  2. 4.1.2 The mysqld-max Extended MySQL Server
  3. 4.1.3 The mysqld_safe Server Startup Script
  4. 4.1.4 The mysql.server Server Startup Script
  5. 4.2 Configuring the MySQL Server
  6. 4.2.2 The Server SQL Mode
  7. 4.2.3 Server System Variables
By: Sams Publishing
Rating: starstarstarstarstar / 28
May 25, 2006

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The server maintains many system variables that indicate how it is configured. All of them have default values. They can be set at server startup using options on the command line or in option files. Most of them can be set at runtime using the SET statement.

Beginning with MySQL 4.0.3, the mysqld server maintains two kinds of variables. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections.

When the server starts, it initializes all global variables to their default values. These defaults can be changed by options specified in option files or on the command line. After the server starts, those global variables that are dynamic can be changed by connecting to the server and issuing a SET GLOBAL var_name statement. To change a global variable, you must have the SUPER privilege.

The server also maintains a set of session variables for each client that connects. The client's session variables are initialized at connect time using the current values of the corresponding global variables. For those session variables that are dynamic, the client can change them by issuing a SET SESSION var_name statement. Setting a session variable requires no special privilege, but a client can change only its own session variables, not those of any other client.

A change to a global variable is visible to any client that accesses that global variable. However, it affects the corresponding session variable that is initialized from the global variable only for clients that connect after the change. It does not affect the session variable for any client that is already connected (not even that of the client that issues the SET GLOBAL statement).

When setting a variable using a startup option, variable values can be given with a suffix of K, M, or G to indicate kilobytes, megabytes, or gigabytes, respectively. For example, the following command starts the server with a key buffer size of 16 megabytes:

mysqld --key_buffer_size=16M

Before MySQL 4.0, use this syntax instead:

mysqld --set-variable=key_buffer_size=16M

The lettercase of suffix letters does not matter; 16M and 16m are equivalent.

At runtime, use the SET statement to set system variables. In this context, suffix letters cannot be used, but the value can take the form of an expression:

mysql> SET sort_buffer_size = 10 * 1024 * 1024;

To specify explicitly whether to set the global or session variable, use the GLOBAL or SESSION options:

mysql> SET GLOBAL sort_buffer_size = 10 * 1024 *
1024;
mysql> SET SESSION sort_buffer_size = 10 * 1024 *
1024;

Without either option, the statement sets the session variable.

The variables that can be set at runtime are listed in Section 4.2.3.1.2, "Dynamic System Variables."

If you want to restrict the maximum value to which a system variable can be set with the SET statement, you can specify this maximum by using an option of the form --maximum-var_name at server startup. For example, to prevent the value of query_cache_size from being increased to more than 32MB at runtime, use the option --maximum-query_cache_size=32M. This feature is available as of MySQL 4.0.2.

You can view system variables and their values by using the SHOW VARIABLES statement. See Section 4.2.3.1, "System Variables," for more information.

mysql> SHOW VARIABLES;
+---------------------------------+------------------------------+
| Variable_name                   | Value                        |
+---------------------------------+------------------------------|
| back_log                        | 50                           |
| basedir                         | /usr/local/mysql             |
| bdb_cache_size                  | 8388572                      |
| bdb_home                        | /usr/local/mysql             |
| bdb_log_buffer_size             | 32768                        |
| bdb_logdir                      |                              |
| bdb_max_lock                    | 10000                        |
| bdb_shared_data                 | OFF                          |
| bdb_tmpdir                      | /tmp/                        |
| bdb_version                     | Sleepycat Software: ...      |
| binlog_cache_size               | 32768                        |
| bulk_insert_buffer_size         | 8388608                      |
| character_set                   | latin1                       |
| character_sets                  | latin1 big5 czech euc_kr     |
| concurrent_insert               | ON                           |
| connect_timeout                 | 5                            |
| convert_character_set           |                              |
| datadir                         | /usr/local/mysql/data/       |
| default_week_format             | 0                            |
| delay_key_write                 | ON                           |
| delayed_insert_limit            | 100                          |
| delayed_insert_timeout          | 300                          |
| delayed_queue_size              | 1000                         |
| flush                           | OFF                          |
| flush_time                      | 0                            |
| ft_boolean_syntax               | + -><()~*:""&|               |
| ft_max_word_len                 | 84                           |
| ft_min_word_len                 | 4                            |
| ft_query_expansion_limit        | 20                           |
| ft_stopword_file                | (built-in)                   |
| have_bdb                        | YES                          |
| have_innodb                     | YES                          |
| have_isam                       | YES                          |
| have_openssl                    | YES                          |
| have_query_cache                | YES                          |
| have_raid                       | NO                           |
| have_symlink                    | DISABLED                     |
| init_file                       |                              |
| innodb_additional_mem_pool_size | 1048576                      |
| innodb_buffer_pool_size         | 8388608                      |
| innodb_data_file_path           | ibdata1:10M:autoextend       |
| innodb_data_home_dir            |                              |
| innodb_fast_shutdown            | ON                           |
| innodb_file_io_threads          | 4                            |
| innodb_flush_log_at_trx_commit  | 1                            |
| innodb_flush_method             |                              |
| innodb_force_recovery           | 0                            |
| innodb_lock_wait_timeout        | 50                           |
| innodb_log_arch_dir             |                              |
| innodb_log_archive              | OFF                          |
| innodb_log_buffer_size          | 1048576                      |
| innodb_log_file_size            | 5242880                      |
| innodb_log_files_in_group       | 2                            |
| innodb_log_group_home_dir       | ./                           |
| innodb_mirrored_log_groups      | 1                            |
| innodb_thread_concurrency       | 8                            |
| interactive_timeout             | 28800                        |
| join_buffer_size                | 131072                       |
| key_buffer_size                 | 16773120                     |
| key_cache_age_threshold         | 300                          |
| key_cache_block_size            | 1024                         |
| key_cache_division_limit        | 100                          |
| language                        | /usr/local/mysql/share/...   |
| large_files_support             | ON                           |
| local_infile                    | ON                           |
| locked_in_memory                | OFF                          |
| log                             | OFF                          |
| log_bin                         | OFF                          |
| log_slave_updates               | OFF                          |
| log_slow_queries                | OFF                          |
| log_update                      | OFF                          |
| log_warnings                    | OFF                          |
| long_query_time                 | 10                           |
| low_priority_updates            | OFF                          |
| lower_case_table_names          | 0                            |
| max_allowed_packet              | 1047552                      |
| max_binlog_cache_size           | 4294967295                   |
| max_binlog_size                 | 1073741824                   |
| max_connect_errors              | 10                           |
| max_connections                 | 100                          |
| max_delayed_threads             | 20                           |
| max_error_count                 | 64                           |
| max_heap_table_size             | 16777216                     |
| max_join_size                   | 4294967295                   |
| max_relay_log_size              | 0                            |
| max_sort_length                 | 1024                         |
| max_tmp_tables                  | 32                           |
| max_user_connections            | 0                            |
| max_write_lock_count            | 4294967295                   |
| myisam_max_extra_sort_file_size | 268435456                    |
| myisam_max_sort_file_size       | 2147483647                   |
| myisam_recover_options          | force                        |
| myisam_repair_threads           | 1                            |
| myisam_sort_buffer_size         | 8388608                      |
| net_buffer_length               | 16384                        |
| net_read_timeout                | 30                           |
| net_retry_count                 | 10                           |
| net_write_timeout               | 60                           |
| open_files_limit                | 1024                         |
| pid_file                        | /usr/local/mysql/name.pid    |
| port                            | 3306                         |
| protocol_version                | 10                           |
| query_cache_limit               | 1048576                      |
| query_cache_size                | 0                            |
| query_cache_type                | ON                           |
| read_buffer_size                | 131072                       |
| read_rnd_buffer_size            | 262144                       |
| rpl_recovery_rank               | 0                            |
| server_id                       | 0                            |
| skip_external_locking           | ON                           |
| skip_networking                 | OFF                          |
| skip_show_database              | OFF                          |
| slave_net_timeout               | 3600                         |
| slow_launch_time                | 2                            |
| socket                          | /tmp/mysql.sock              |
| sort_buffer_size                | 2097116                      |
| sql_mode                        |                              |
| table_cache                     | 64                           |
| table_type                      | MYISAM                       |
| thread_cache_size               | 3                            |
| thread_stack                    | 131072                       |
| timezone                        | EEST                         |
| tmp_table_size                  | 33554432                     |
| tmpdir                          | /tmp/:/mnt/hd2/tmp/          |
| tx_isolation                    | READ-COMMITTED               |
| version                         | 4.0.4-beta                   |
| wait_timeout                    | 28800                        |
+---------------------------------+------------------------------+

Most system variables are described here. Variables with no version indicated have been present since at least MySQL 3.22. InnoDB system variables are listed in Section 9.5, "InnoDB Startup Options."

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.

Information on tuning these variables can be found in Section 6.5.2, "Tuning Server Parameters."

  • ansi_mode

    This is ON if mysqld was started with --ansi. See Section 1.8.3, "Running MySQL in ANSI Mode." This variable was added in MySQL 3.23.6 and removed in 3.23.41. See the description for --sql-mode.

  • back_log

    The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets very many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time.

    In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen() system call should have more details. Check your OS documentation for the maximum value for this variable. Attempting to set back_log higher than your operating system limit will be ineffective.

  • basedir

    The MySQL installation base directory. This variable can be set with the --basedir option.

  • bdb_cache_size

    The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you don't use BDB tables, you should start mysqld with --skip-bdb to not waste memory for this cache. This variable was added in MySQL 3.23.14.

  • bdb_home

    The base directory for BDB tables. This should be assigned the same value as the datadir variable. This variable was added in MySQL 3.23.14.

  • bdb_log_buffer_size

    The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you don't use BDB tables, you should set this to 0 or start mysqld with --skip-bdb to not waste memory for this cache. This variable was added in MySQL 3.23.31.

  • bdb_logdir

    The directory where the BDB storage engine writes its log files. This variable can be set with the --bdb-logdir option. This variable was added in MySQL 3.23.14.

  • bdb_max_lock

    The maximum number of locks you can have active on a BDB table (10,000 by default). You should increase this if errors such as the following occur when you perform long transactions or when mysqld has to examine many rows to calculate a query:

    bdb: Lock table is out of available locks
    Got error 12 from ...

    This variable was added in MySQL 3.23.29.

  • bdb_shared_data

    This is ON if you are using --bdb-shared-data. This variable was added in MySQL 3.23.29.

  • bdb_tmpdir

    The value of the --bdb-tmpdir option. This variable was added in MySQL 3.23.14.

  • bdb_version

    The BDB storage engine version. This variable was added in MySQL 3.23.31.

  • binlog_cache_size

    The size of the cache to hold the SQL statements for the binary log during a transaction. A binary log cache is allocated for each client if the server supports any transactional storage engines and, starting from MySQL 4.1.2, if the server has binary log enabled (--log-bin option). If you often use big, multiple-statement transactions, you can increase this to get more performance. The Binlog_cache_use and Binlog_cache_disk_use status variables can be useful for tuning the size of this variable. This variable was added in MySQL 3.23.29. See Section 4.8.4, "The Binary Log."

  • bulk_insert_buffer_size

    MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA INFILE. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. Note: This cache is used only when adding data to a non-empty table. The default value is 8MB. This variable was added in MySQL 4.0.3. This variable previously was named myisam_bulk_insert_tree_size.

  • character_set

    The default character set. This variable was added in MySQL 3.23.3, then removed in MySQL 4.1.1 and replaced by the various character_set_xxx variables.

  • character_set_client

    The character set for statements that arrive from the client. This variable was added in MySQL 4.1.1.

  • character_set_connection

    The character set used for literals that do not have a character set introducer, for some functions, and for number-to-string conversion. This variable was added in MySQL 4.1.1.

  • character_set_database

    The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as character_set_server. This variable was added in MySQL 4.1.1.

  • character_set_results

    The character set used for returning query results to the client. This variable was added in MySQL 4.1.1.

  • character_set_server

    The server default character set. This variable was added in MySQL 4.1.1.

  • character_set_system

    The character set used by the server for storing identifiers. The value is always utf8. This variable was added in MySQL 4.1.1.

  • character_sets

    The supported character sets. This variable was added in MySQL 3.23.15.

  • collation_connection

    The collation of the connection character set. This variable was added in MySQL 4.1.1.

  • collation_database

    The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as collation_server. This variable was added in MySQL 4.1.1.

  • collation_server

    The server default collation. This variable was added in MySQL 4.1.1.

  • concurrent_inserts

    If ON (the default), MySQL allows INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle. You can turn this option off by starting mysqld with --safe or --skip-new. This variable was added in MySQL 3.23.7.

  • connect_timeout

    The number of seconds the mysqld server waits for a connect packet before responding with Bad handshake.

  • datadir

    The MySQL data directory. This variable can be set with the --datadir option.

  • default_week_format

    The default mode value to use for the WEEK() function. This variable is available as of MySQL 4.0.14.

  • delay_key_write

    This option applies only to MyISAM tables. It can have one of the following values to affect handling of the DELAY_KEY_WRITE table option that can be used in CREATE TABLE statements.

    Option Description 
    OFF DELAYED_KEY_WRITE is ignored. 
    ON MySQL honors the DELAY_KEY_WRITE option for CREATE TABLE. This is the default value. 
    ALL All new opened tables are treated as if they were created with the DELAY_KEY_WRITE option enabled. 

    If DELAY_KEY_WRITE is enabled, this means that the key buffer for tables with this option are not flushed on every index update, but only when a table is closed. This will speed up writes on keys a lot, but if you use this feature, you should add automatic checking of all MyISAM tables by starting the server with the --myisam-recover option (for example, --myisam-recover=BACKUP, FORCE). See Section 4.2.1, "mysqld Command-Line Options," and Section 8.1.1, "MyISAM Startup Options."

    Note that --external-locking doesn't offer any protection against index corruption for tables that use delayed key writes.

    This variable was added in MySQL 3.23.8.

  • delayed_insert_limit

    After inserting delayed_insert_limit delayed rows, the INSERT DELAYED handler thread checks whether there are any SELECT statements pending. If so, it allows them to execute before continuing to insert delayed rows.

  • delayed_insert_timeout

    How long an INSERT DELAYED handler thread should wait for INSERT statements before terminating.

  • delayed_queue_size

    How many rows to queue when handling INSERT DELAYED statements. If the queue becomes full, any client that issues an INSERT DELAYED statement will wait until there is room in the queue again.

  • flush

    This is ON if you have started mysqld with the --flush option. This variable was added in MySQL 3.22.9.

  • flush_time

    If this is set to a non-zero value, all tables will be closed every flush_time seconds to free up resources and sync unflushed data to disk. We recommend this option only on Windows 9x or Me, or on systems with minimal resources available. This variable was added in MySQL 3.22.18.

  • ft_boolean_syntax

    The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. This variable was added in MySQL 4.0.1.

    The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:

    • Operator function is determined by position within the string.

    • The replacement value must be 14 characters.

    • Each character must be an ASCII non-alphanumeric character.

    • Either the first or second character must be a space.

    • No duplicates are allowed except the phrase quoting operators in positions 11 and 12. These two characters are not required to be the same, but they are the only two that may be.

    • Positions 10, 13, and 14 (which by default are set to ':', '&', and '|') are reserved for future extensions.

  • ft_max_word_len

    The maximum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.

    Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.

  • ft_min_word_len

    The minimum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.

    Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.

  • ft_query_expansion_limit

    The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION. This variable was added in MySQL 4.1.1.

  • ft_stopword_file

    The file from which to read the list of stopwords for full-text searches. All the words from the file are used; comments are not honored. By default, a built-in list of stopwords is used (as defined in the myisam/ft_static.c file). Setting this variable to the empty string ('') disables stopword filtering. This variable was added in MySQL 4.0.10.

    Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.

  • group_concat_max_len

    The maximum allowed result length for the GROUP_CONCAT() function. This variable was added in MySQL 4.1.0.

  • have_bdb

    YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used. This variable was added in MySQL 3.23.30.

  • have_innodb

    YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used. This variable was added in MySQL 3.23.37.

  • have_isam

    YES if mysqld supports ISAM tables. DISABLED if --skip-isam is used. This variable was added in MySQL 3.23.30.

  • have_raid

    YES if mysqld supports the RAID option. This variable was added in MySQL 3.23.30.

  • have_openssl

    YES if mysqld supports SSL (encryption) of the client/server protocol. This variable was added in MySQL 3.23.43.

  • init_connect

    A string to be executed by the server for each client that connects. The string consists of one or more SQL statements. To specify multiple statements, separate them by semicolon characters. For example, each client begins by default with autocommit mode enabled. There is no global server variable to specify that autocommit should be disabled by default, but init_connect can be used to achieve the same effect:

    SET GLOBAL init_connect='SET AUTOCOMMIT=0';

    This variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:

    [mysqld]
    init_connect='SET AUTOCOMMIT=0'

    This variable was added in MySQL 4.1.2.

  • init_file

    The name of the file specified with the --init-file option when you start the server. This is a file containing SQL statements that you want the server to execute when it starts. Each statement must be on a single line and should not include comments. This variable was added in MySQL 3.23.2.

  • init_slave

    This variable is similar to init_connect, but is a string to be executed by a slave server each time the SQL thread starts. The format of the string is the same as for the init_connect variable. This variable was added in MySQL 4.1.2.

  • innodb_xxx

    The InnoDB system variables are listed in Section 9.5, "InnoDB Startup Options."

  • interactive_timeout

    The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.

  • join_buffer_size

    The size of the buffer that is used for full joins (joins that do not use indexes). The buffer is allocated one time for each full join between two tables. Increase this value to get a faster full join when adding indexes is not possible. (Normally the best way to get fast joins is to add indexes.)

  • key_buffer_size

    Index blocks for MyISAM and ISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.

    Increase the value to get better index handling (for all reads and multiple writes) to as much as you can afford. Using a value that is 25% of total memory on a machine that mainly runs MySQL is quite common. However, if you make the value too large (for example, more than 50% of your total memory) your system might start to page and become extremely slow. MySQL relies on the operating system to perform filesystem caching for data reads, so you must leave some room for the filesystem cache.

    For even more speed when writing many rows at the same time, use LOCK TABLES.

    You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables.

    The Key_reads/Key_read_requests ratio should normally be less than 0.01. The Key_writes/Key_write_requests ratio is usually near 1 if you are using mostly updates and deletes, but might be much smaller if you tend to do updates that affect many rows at the same time or if you are using the DELAY_KEY_WRITE table option.

    The fraction of the key buffer in use can be determined using key_buffer_size in conjunction with the Key_blocks_used status variable and the buffer block size. From MySQL 4.1.1 on, the buffer block size is available from the key_cache_block_size server variable. The fraction of the buffer in use is:

    (Key_blocks_used * key_cache_block_size) /
    key_buffer_size
  • Before MySQL 4.1.1, key cache blocks are 1024 bytes, so the fraction of the key buffer in use is:

    (Key_blocks_used * 1024) / key_buffer_size

    See Section 6.4.6, "The MyISAM Key Cache."

  • key_cache_age_threshold

    This value controls the demotion of buffers from the hot sub-chain of a key cache to the warm sub-chain. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. This variable was added in MySQL 4.1.1. See Section 6.4.6, "The MyISAM Key Cache."

  • key_cache_block_size

    The size in bytes of blocks in the key cache. The default value is 1024. This variable was added in MySQL 4.1.1. See Section 6.4.6, "The MyISAM Key Cache."

  • key_cache_division_limit

    The division point between the hot and warm sub-chains of the key cache buffer chain. The value is the percentage of the buffer chain to use for the warm sub-chain. Allowable values range from 1 to 100. The default value is 100. This variable was added in MySQL 4.1.1. See Section 6.4.6, "The MyISAM Key Cache."

  • language

    The language used for error messages.

  • large_file_support

    Whether mysqld was compiled with options for large file support. This variable was added in MySQL 3.23.28.

  • local_infile

    Whether LOCAL is supported for LOAD DATA INFILE statements. This variable was added in MySQL 4.0.3.

  • locked_in_memory

    Whether mysqld was locked in memory with --memlock. This variable was added in MySQL 3.23.25.

  • log

    Whether logging of all queries to the general query log is enabled. See Section 4.8.2, "The General Query Log."

  • log_bin

    Whether the binary log is enabled. This variable was added in MySQL 3.23.14. See Section 4.8.4, "The Binary Log."

  • log_slave_updates

    Whether updates received by a slave server from a master server should be logged to the slave's own binary log. Binary logging must be enabled on the slave for this to have any effect. This variable was added in MySQL 3.23.17. See Section 5.8, "Replication Startup Options."

  • log_slow_queries

    Whether slow queries should be logged. "Slow" is determined by the value of the long_query_time variable. This variable was added in MySQL 4.0.2. See Section 4.8.5, "The Slow Query Log."

  • log_update

    Whether the update log is enabled. This variable was added in MySQL 3.22.18. Note that the binary log is preferable to the update log, which is unavailable as of MySQL 5.0. See Section 4.8.3, "The Update Log."

  • long_query_time

    If a query takes longer than this many seconds, the Slow_queries status variable is incremented. If you are using the --log-slow-queries option, the query is logged to the slow query log file. This value is measured in real time, not CPU time, so a query that is under the threshold on a lightly loaded system might be above the threshold on a heavily loaded one. See Section 4.8.5, "The Slow Query Log."

  • low_priority_updates

    If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. This variable previously was named sql_low_priority_updates. It was added in MySQL 3.22.5.

  • lower_case_table_names

    If set to 1, table names are stored in lowercase on disk and table name comparisons are not case sensitive. This variable was added in MySQL 3.23.6. If set to 2 (new in 4.0.18), table names are stored as given but compared in lowercase. From MySQL 4.0.2, this option also applies to database names. From 4.1.1, it also applies to table aliases.

    You should not set this variable to 0 if you are running MySQL on a system that does not have case-sensitive filenames (such as Windows or Mac OS X). New in 4.0.18: If this variable is 0 and the filesystem on which the data directory is located does not have case-sensitive filenames, MySQL automatically sets lower_case_table_names to 2.

  • max_allowed_packet

    The maximum size of one packet. The message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small, to catch big (possibly wrong) packets. You must increase this value if you are using big BLOB columns. It should be as big as the biggest BLOB you want to use. The protocol limit for max_allowed_packet is 16MB before MySQL 4.0 and 1GB thereafter.

  • max_binlog_cache_size

    If a multiple-statement transaction requires more than this amount of memory, you will get the error Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage. This variable was added in MySQL 3.23.29.

  • max_binlog_size

    If a write to the binary log exceeds the given value, rotate the binary logs. You cannot set this variable to more than 1GB or to less than 4096 bytes. (The minimum before MYSQL 4.0.14 is 1024 bytes.) The default value is 1GB. This variable was added in MySQL 3.23.33.

    Note if you are using transactions: A transaction is written in one chunk to the binary log, hence it is never split between several binary logs. Therefore, if you have big transactions, you might see binary logs bigger than max_binlog_size.

    If max_relay_log_size is 0, the value of max_binlog_size applies to relay logs as well. max_relay_log_size was added in MySQL 4.0.14.

  • max_connect_errors

    If there are more than this number of interrupted connections from a host, that host is blocked from further connections. You can unblock blocked hosts with the FLUSH HOSTS statement.

  • max_connections

    The number of simultaneous client connections allowed. Increasing this value increases the number of file descriptors that mysqld requires. See Section 6.4.8, "How MySQL Opens and Closes Tables," for comments on file descriptor limits. Also see Section A.2.6, "Too many connections."

  • max_delayed_threads

    Don't start more than this number of threads to handle INSERT DELAYED statements. If you try to insert data into a new table after all INSERT DELAYED threads are in use, the row will be inserted as if the DELAYED attribute wasn't specified. If you set this to 0, MySQL never creates a thread to handle DELAYED rows; in effect, this disables DELAYED entirely. This variable was added in MySQL 3.23.0.

  • max_error_count

    The maximum number of error, warning, and note messages to be stored for display by SHOW ERRORS or SHOW WARNINGS. This variable was added in MySQL 4.1.0.

  • max_heap_table_size

    This variable sets the maximum size to which MEMORY (HEAP) tables are allowed to grow. The value of the variable is used to calculate MEMORY table MAX_ROWS values. Setting this variable has no effect on any existing MEMORY table, unless the table is re-created with a statement such as CREATE TABLE or TRUNCATE TABLE, or altered with ALTER TABLE. This variable was added in MySQL 3.23.0.

  • max_insert_delayed_threads

    This variable is a synonym for max_delayed_threads. It was added in MySQL 4.0.19.

  • max_join_size

    Don't allow SELECT statements that probably will need to examine more than max_join_size row combinations or are likely to do more than max_join_size disk seeks. By setting this value, you can catch SELECT statements where keys are not used properly and that would probably take a long time. Set it if your users tend to perform joins that lack a WHERE clause, that take a long time, or that return millions of rows.

    Setting this variable to a value other than DEFAULT resets the SQL_BIG_SELECTS value to 0. If you set the SQL_BIG_SELECTS value again, the max_join_size variable is ignored.

    If a query result already is in the query cache, no result size check is performed, because the result has already been computed and it does not burden the server to send it to the client.

    This variable previously was named sql_max_join_size.

  • max_relay_log_size

    If a write by a replication slave to its relay log exceeds the given value, rotate the relay log. This variable enables you to put different size constraints on relay logs and binary logs. However, setting the variable to 0 makes MySQL use max_binlog_size for both binary logs and relay logs. You must set max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. This variable was added in MySQL 4.0.14. See Section 5.3, "Replication Implementation Details."

  • max_seeks_for_key

    Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer will assume that no more than this number of key seeks will be required when searching for matching rows in a table by scanning a key, regardless of the actual cardinality of the key. By setting this to a low value (100?), you can force MySQL to prefer keys instead of table scans. This variable was added in MySQL 4.0.14.

  • max_sort_length

    The number of bytes to use when sorting BLOB or TEXT values. Only the first max_sort_length bytes of each value are used; the rest are ignored.

  • max_tmp_tables

    The maximum number of temporary tables a client can keep open at the same time. (This option doesn't yet do anything.)

  • max_user_connections

    The maximum number of simultaneous connections allowed to any given MySQL account. A value of 0 means "no limit." This variable was added in MySQL 3.23.34.

  • max_write_lock_count

    After this many write locks, allow some read locks to run in between. This variable was added in MySQL 3.23.7.

  • myisam_data_pointer_size

    Default pointer size in bytes to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 8. The default value is 4. This variable was added in MySQL 4.1.2. See Section A.2.11, "The table is full."

  • myisam_max_extra_sort_file_size

    If the temporary file used for fast MyISAM index creation would be larger than using the key cache by the amount specified here, prefer the key cache method. This is mainly used to force long character keys in large tables to use the slower key cache method to create the index. This variable was added in MySQL 3.23.37. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.

  • myisam_max_sort_file_size

    The maximum size of the temporary file MySQL is allowed to use while re-creating a MyISAM index (during REPAIR TABLE, ALTER TABLE, or LOAD DATA INFILE). If the file size would be bigger than this value, the index will be created using the key cache instead, which is slower. This variable was added in MySQL 3.23.37. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.

  • myisam_recover_options

    The value of the --myisam-recover option. This variable was added in MySQL 3.23.36.

  • myisam_repair_threads

    If this value is greater than 1, MyISAM table indexes are created in parallel (each index in its own thread) during the Repair by sorting process. The default value is 1. Note: Multi-threaded repair is still alpha quality code. This variable was added in MySQL 4.0.13.

  • myisam_sort_buffer_size

    The buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE. This variable was added in MySQL 3.23.16.

  • named_pipe

    On Windows, indicates whether the server supports connections over named pipes. This variable was added in MySQL 3.23.50.

  • net_buffer_length

    The communication buffer is reset to this size between queries. This should not normally be changed, but if you have very little memory, you can set it to the expected length of SQL statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to max_allowed_packet bytes.

  • net_read_timeout

    The number of seconds to wait for more data from a connection before aborting the read. When the server is reading from the client, net_read_timeout is the timeout value controlling when to abort. When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort. See also slave_net_timeout. This variable was added in MySQL 3.23.20.

  • net_retry_count

    If a read on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads. This variable was added in MySQL 3.23.7.

  • net_write_timeout

    The number of seconds to wait for a block to be written to a connection before aborting the write. See also net_read_timeout. This variable was added in MySQL 3.23.20.

  • open_files_limit

    The number of files that the operating system allows mysqld to open. This is the real value allowed by the system and might be different from the value you gave mysqld as a startup option. The value is 0 on systems where MySQL can't change the number of open files. This variable was added in MySQL 3.23.20.

  • pid_file

    The pathname of the process ID (PID) file. This variable can be set with the --pid-file option. This variable was added in MySQL 3.23.23.

  • port

    The port on which the server listens for TCP/IP connections. This variable can be set with the --port option.

  • protocol_version

    The version of the client/server protocol used by the MySQL server. This variable was added in MySQL 3.23.18.

  • query_alloc_block_size

    The allocation size of memory blocks that are allocated for objects created during query parsing and execution. If you have problems with memory fragmentation, it might help to increase this a bit. This variable was added in MySQL 4.0.16.

  • query_cache_limit

    Don't cache results that are bigger than this. The default value is 1MB. This variable was added in MySQL 4.0.1.

  • query_cache_min_res_unit

    The minimum size for blocks allocated by the query cache. The default value is 4KB. Tuning information for this variable is given in Section 4.10.3, "Query Cache Configuration." This variable is present from MySQL 4.1.

  • query_cache_size

    The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. Note that this amount of memory will be allocated even if query_cache_type is set to 0. This variable was added in MySQL 4.0.1.

  • query_cache_type

    Set query cache type. Setting the GLOBAL value sets the type for all clients that connect thereafter. Individual clients can set the SESSION value to affect their own use of the query cache.

    Option

    Description

    0 or OFF

    Don't cache or retrieve results. Note that this will not deallocate the query cache buffer. To do that, you should set query_cache_size to 0.

    1 or ON

    Cache all query results except for those that begin with SELECT SQL_NO_CACHE.

    2 or DEMAND

    Cache results only for queries that begin with SELECT SQL_CACHE.


    This variable was added in MySQL 4.0.3.

  • query_cache_wlock_invalidate

    Normally, when one client acquires a WRITE lock on a MyISAM table, other clients are not blocked from issuing queries for the table if the query results are present in the query cache. Setting this variable to 1 causes acquisition of a WRITE lock for a table to invalidate any queries in the query cache that refer to the table. This forces other clients that attempt to access the table to wait while the lock is in effect. This variable was added in MySQL 4.0.19.

  • query_prealloc_size

    The size of the persistent buffer used for query parsing and execution. This buffer is not freed between queries. If you are running complex queries, a larger query_prealloc_size value might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations.

    This variable was added in MySQL 4.0.16.

  • range_alloc_block_size

    The size of blocks that are allocated when doing range optimization. This variable was added in MySQL 4.0.16.

  • read_buffer_size

    Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you might want to increase this value. This variable was added in MySQL 4.0.3. Previously, it was named record_buffer.

  • read_only

    When the variable is set to ON for a replication slave server, it causes the slave to allow no updates except from slave threads or from users with the SUPER privilege. This can be useful to ensure that a slave server accepts no updates from clients. This variable was added in MySQL 4.0.14.

  • read_rnd_buffer_size

    When reading rows in sorted order after a sort, the rows are read through this buffer to avoid disk seeks. Setting the variable to a large value can improve ORDER BY performance by a lot. However, this is a buffer allocated for each client, so you should not set the global variable to a large value. Instead, change the session variable only from within those clients that need to run large queries. This variable was added in MySQL 4.0.3. Previously, it was named record_rnd_buffer.

  • safe_show_database

    Don't show databases for which the user has no database or table privileges. This can improve security if you're concerned about people being able to see what databases other users have. See also skip_show_database.

    This variable was removed in MySQL 4.0.5. Instead, use the SHOW DATABASES privilege to control access by MySQL accounts to database names.

  • secure_auth

    If the MySQL server has been started with the --secure-auth option, it blocks connections from all accounts that have passwords stored in the old (pre-4.1) format. In that case, the value of this variable is ON, otherwise it is OFF.

    You should enable this option if you want to prevent all usage of passwords in old format (and hence insecure communication over the network). This variable was added in MySQL 4.1.1.

    Server startup will fail with an error if this option is enabled and the privilege tables are in pre-4.1 format.

    When used as a client-side option, the client refuses to connect to a server if the server requires a password in old format for the client account.

  • server_id

    The value of the --server-id option. It is used for master and slave replication servers. This variable was added in MySQL 3.23.26.

  • skip_external_locking

    This is OFF if mysqld uses external locking. This variable was added in MySQL 4.0.3. Previously, it was named skip_locking.

  • skip_networking

    This is ON if the server allows only local (non-TCP/IP) connections. On Unix, local connections use a Unix socket file. On Windows, local connections use a named pipe. On NetWare, only TCP/IP connections are supported, so do not set this variable to ON. This variable was added in MySQL 3.22.23.

  • skip_show_database

    This prevents people from using the SHOW DATABASES statement if they don't have the SHOW DATABASES privilege. This can improve security if you're concerned about people being able to see what databases other users have. See also safe_show_database. This variable was added in MySQL 3.23.4. As of MySQL 4.0.2, its effect also depends on the SHOW DATABASES privilege: If the variable value is ON, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database.

  • slave_net_timeout

    The number of seconds to wait for more data from a master/slave connection before aborting the read. This variable was added in MySQL 3.23.40.

  • slow_launch_time

    If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable. This variable was added in MySQL 3.23.15.

  • socket

    On Unix, this is the Unix socket file used for local client connections. On Windows, this is the name of the named pipe used for local client connections.

  • sort_buffer_size

    Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster ORDER BY or GROUP BY operations. See Section A.4.4, "Where MySQL Stores Temporary Files."

  • sql_mode

    The current server SQL mode. This variable was added in MySQL 3.23.41. See Section 4.2.2, "The Server SQL Mode."

  • storage_engine

    This variable is a synonym for table_type. It was added in MySQL 4.1.2.

  • table_cache

    The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. You can check whether you need to increase the table cache by checking the Opened_tables status variable. See Section 4.2.4, "Server Status Variables." If the value of Opened_tables is large and you don't do FLUSH TABLES a lot (which just forces all tables to be closed and reopened), then you should increase the value of the table_cache variable.

    For more information about the table cache, see Section 6.4.8, "How MySQL Opens and Closes Tables."

  • table_type

    The default table type (storage engine). To set the table type at server startup, use the --default-table-type option. This variable was added in MySQL 3.23.0. See Section 4.2.1, "mysqld Command-Line Options."

  • thread_cache_size

    How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there aren't already thread_cache_size threads there. Requests for threads are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. (Normally this doesn't give a notable performance improvement if you have a good thread implementation.) By examining the difference between the Connections and Threads_created status variables (see Section 4.2.4, "Server Status Variables," for details) you can see how efficient the thread cache is. This variable was added in MySQL 3.23.16.

  • thread_concurrency

    On Solaris, mysqld calls thr_setconcurrency() with this value. This function allows applications to give the threads system a hint about the desired number of threads that should be run at the same time. This variable was added in MySQL 3.23.7.

  • thread_stack

    The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. The default is large enough for normal operation. See Section 6.1.4, "The MySQL Benchmark Suite."

  • timezone

    The time zone for the server. This is set from the TZ environment variable when mysqld is started. The time zone also can be set by giving a --timezone argument to mysqld_safe. This variable was added in MySQL 3.23.15. See Section A.4.6, "Time Zone Problems."

  • tmp_table_size

    If an in-memory temporary table exceeds this size, MySQL automatically converts it to an on-disk MyISAM table. Increase the value of tmp_table_size if you do many advanced GROUP BY queries and you have lots of memory.

  • tmpdir

    The directory used for temporary files and temporary tables. Starting from MySQL 4.1, this variable can be set to a list of several paths that are used in round-robin fashion. Paths should be separated by colon characters (':') on Unix and semicolon characters (';') on Windows, NetWare, and OS/2.

    This feature can be used to spread the load between several physical disks. If the MySQL server is acting as a replication slave, you should not set tmpdir to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication will fail.

    This variable was added in MySQL 3.22.4.

  • transaction_alloc_block_size

    The allocation size of memory blocks that are allocated for storing queries that are part of a transaction to be stored in the binary log when doing a commit. This variable was added in MySQL 4.0.16.

  • transaction_prealloc_size

    The size of the persistent buffer for transaction_alloc_blocks that is not freed between queries. By making this big enough to fit all queries in a common transaction, you can avoid a lot of malloc() calls. This variable was added in MySQL 4.0.16.

  • tx_isolation

    The default transaction isolation level. This variable was added in MySQL 4.0.3.

  • version

    The version number for the server.

  • wait_timeout

    The number of seconds the server waits for activity on a non-interactive connection before closing it.

    On thread startup, the session wait_timeout value is initialized from the global wait_timeout value or from the global interactive_timeout value, depending on the type of client (as defined by the CLIENT_INTERACTIVE connect option to mysql_real_connect()). See also interactive_timeout.

Please check back next week for the continuation of this article.



 
 
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